Pad grid array packages are becoming a popular form of integrated circuit packaging; as they provide for a large number of pads in a small package. Pad grid arrays are formed by mounting a semiconductor device on a substrate containing an array of pads on the bottom of the substrate. The substrate is registered in a mold cavity by locating the edges of the substrate against registration bumps in the mold cavity. A plastic material is then transfer-molded about the semiconductor device to form a completed package. After the molding is completed, the registration bumps appear in the finished pad grid array package as indentations or ribs.
Some pad grid array assemblies are made from low-cost materials such as epoxy-glass or polyester-glass printed circuit board substrates. The fabrication of these substrates shares most of the same assembly processes and techniques with conventional printed circuit board manufacturing processes. Other types of pad grid array assemblies employ alumina ceramic substrates that more closely match the expansion co-efficient of the semiconductor device, but these substrates are more fragile than the printed circuit board substrates. Similar substrate materials such as aluminum nitride or beryllium oxide are also useful as substrate materials due to their attractive thermal conductivity and electrical properties. However, all these ceramic materials are too fragile to be used in a transfer molded package. During the molding operation the high clamping pressure exerted on the substrate by the mold press fractures the fragile substrates.
Another problem experienced in transfer molding is the need for a dedicated mold tool for each size of semiconductor device used. As the size of the device and substrate changes, the sizes of the openings and registration bumps in the mold cavity must change in order to accommodate the various sizes. In laboratory or pilot plant situations where a large number of different sized semiconductors are being molded, the inventory of mold sizes becomes excessive, and the time required to change the mold cavities becomes prohibitive. It would be desirable to have a method of making a pad array chip carrier that only uses a single mold cavity that would accommodate a number of different size semiconductors and substrates, and would also be able to be used in molding a package made with a fragile ceramic substrate.